Light emitting device

ABSTRACT

A light emitting device can be used for light emitting diodes and laser diodes. The light emitting device includes a substrate, a first semiconductor layer on the substrate, a second semiconductor layer on the first semiconductor layer, and a multi-quantum well structure including at least one well layer and at least one barrier layer between the first and second semiconductor layers. A carrier trap portion is formed in at least one layer within the multi-quantum well structure. The carrier trap portion has a band-gap energy that gradually decreases from a periphery of the carrier trap portion to a center thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.12/486,267, filed Jun. 17, 2009, and claims the benefit under 35 U.S.A.§119 of U.S. Provisional Application No. 61/158,184, filed on Mar. 6,2009, which are all hereby incorporated by reference for all purposes asif fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to light emitting devices that can beused for light emitting diodes (LEDs) and laser diodes (LDs). Moreparticularly, the present disclosure relates to a light emitting devicethat includes at least one carrier trap portion in at least one layerwithin a multi-quantum well structure.

2. Discussion of the Background

Group-III nitrides, such as GaN, AN, InGaN, and the like, have goodthermal stability and direct transition type energy-band structure, andhave received attention in recent years as materials for blue and UVlight emitting diodes and laser diodes. Particularly, InGaN compoundsemiconductors are attracting attention for their narrow band-gapenergy. LEDs employing GaN-based compound semiconductors have variousapplications including full color flat panel displays, light sources ofbacklight units, signal lamps, interior lighting, high-definition lightsources, high-resolution output systems, optical communications, and thelike.

Generally, the LED includes an n-type semiconductor layer, a p-typesemiconductor layer, and an active region interposed between the n-typeand p-type semiconductor layers. The n-type and p-type semiconductorlayers may be formed of Group-III nitride semiconductor layers, forexample, (Al, In, Ga)N-based compound semiconductor layers. The activeregion may have a single quantum well structure having a single welllayer or a multi-quantum well structure having multiple wells andbarrier layers. The multi-quantum well structure may include InGaN-welllayers and GaN-barrier layers alternately stacked on top of each other.The InGaN-well layer may be formed of n-type or p-type semiconductorlayer, which has a smaller band gap than the barrier layer, so that aquantum well layer can be formed to permit recombination of an electronand a hole therein.

The Group-III nitride semiconductor layer is grown on a heterogeneoussubstrate having a hexagonal structure, such as a sapphire substrate ora silicon carbide substrate, via metal organic chemical vapor depositionand the like. However, when the Group-III nitride semiconductor layer isgrown on the heterogeneous substrate, the semiconductor layer undergoescracking or warpage and dislocations due to differences in latticeconstant and thermal expansion is coefficient between the semiconductorlayer and the substrate.

To prevent such problems, a buffer layer is formed on the substratebefore growing the semiconductor layer, so that crystal defects can besubstantially prevented in the semiconductor layer grown on the bufferlayer. Nevertheless, the active layer still has a high density ofcrystal defects, thereby providing a severe obstruction in applicationof the Group-III nitride semiconductor layer. Further, the crystaldefects such as dislocations in the active region trap carriersintroduced into the active region and do not emit light, thereby actingas a non-radiative center and significantly deteriorating internalquantum efficiency of the LED.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light emitting deviceconfigured to prevent a reduction in internal quantum efficiency whichis caused by crystal defects such as dislocations in an active region.

Another object of the present invention is to provide a light emittingdevice configured to improve crystal quality of a multi-quantum wellstructure.

In accordance with one aspect, a light emitting device includes asubstrate; a first semiconductor layer on the substrate; a secondsemiconductor layer on the first semiconductor layer; and amulti-quantum well structure including at least one well layer and atleast one barrier layer between the first semiconductor layer and thesecond semiconductor layer, at least one layer within the multi-quantumwell structure including at least one carrier trap portion formedtherein, the at least one carrier trap portion having a band-gap energydecreasing from a periphery of the carrier trap portion to a center ofthe carrier trap portion.

The carrier trap portion may have a band-gap energy gradually decreasingin a is straight line shape, in a step shape, or in a curved line shapefrom a periphery of the carrier trap portion to a center of the carriertrap portion.

The carrier trap portion may be formed in the well layer within themulti-quantum well structure. For example, the carrier trap portion maybe embedded in the well layer.

The layer including the carrier trap portion may contain indium. Forexample, the layer including the carrier trap portion may be composed ofan AlInGaN-based compound semiconductor or an AlInGaP-based compoundsemiconductor.

The light emitting device may further include an indium evaporationpreventing layer in the multi-quantum well structure to preventevaporation of indium from the layer including the carrier trap portion.

The carrier trap portion may contain indium in an amount graduallyincreasing from the periphery of the carrier trap portion to the centerthereof. The center of the carrier trap portion may contain at least 2%or more indium than an outermost region of the carrier trap portion.

The layer including the carrier trap portion may be the well layer andthe barrier layer may contain aluminum (Al). Here, the Al content of thebarrier layer may be adjusted to generate tensile strength capable ofoffsetting compressive stress generated in the layer including thecarrier trap portion. The barrier layer, which contains aluminum, may becomposed of an AlGaInN-based compound semiconductor or an AlGaInP-basedcompound semiconductor.

The carrier trap portion may be formed simultaneously with the layerincluding the carrier trap portion while the layer is grown. The carriertrap portions may be distributed at a higher density than a dislocationdensity of the layer including the carrier trap portion.

The carrier trap portion may have a size of 1˜10 nm. For example, thecarrier trap is portion may have a size of 2˜5 nm.

The light emitting device may further include a carrier trap clusterformed by clustering at least two carrier trap portions. The carriertrap cluster may be separated a distance of at least 20 nm or more fromanother adjacent carrier trap cluster. For example, the carrier trapcluster may be separated a distance of at least 40˜120 nm from anotheradjacent carrier trap cluster.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a stack structure of alight emitting device according to one embodiment of the presentinvention;

FIG. 2 is a TEM image of a multi-quantum well (MQW) having a carriertrap portion according to one embodiment of the present invention;

FIG. 3 is a diagram of an MQW having a carrier trap portion according toone embodiment of the present invention;

FIG. 4 (a) is energy band diagrams taken along lines I(a)-I(b) andII(a)-II(b) of FIG. 3, FIGS. 4 (b) and (c) are another possible energyband diagrams taken along lines I(a)-I(b) of FIG. 3;

FIG. 5 is a TEM image of a multi-quantum well (MQW) having a carriertrap to cluster according to another embodiment of the presentinvention;

FIG. 6 is an APT image of a multi-quantum well (MQW) having a carriertrap cluster according to another embodiment of the present invention;and

FIG. 7 is a diagram illustrating compressive stress generated in acarrier trap portion and tensile stress generated in a barrier layer tooffset the compressive stress.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Thefollowing embodiments are given by way of illustration to provide athorough understanding of the invention to those skilled in the art.Hence, it should be understood that other embodiments will be evidentbased on the present disclosure, and that modifications and changes maybe made without departing from the scope of the present disclosure.Likewise, it should be noted that the drawings are not to precise scaleand some of the dimensions are exaggerated for clarity of description inthe drawings. In addition, like elements are denoted by like referencenumerals throughout the specification and drawings.

FIG. 1 illustrates a stack structure of a light emitting deviceaccording to one embodiment of the present invention.

Referring to FIG. 1, a first semiconductor layer 15 is located on asubstrate 11. Here, a buffer layer 13 may be formed between the firstsemiconductor layer 15 and the substrate 11 to alleviate latticemismatch therebetween. For example, the buffer layer 13 is composed ofGaN or AN. The substrate 11 may be composed of, but is not limited to,sapphire, spinel, or silicon carbide (SiC). The first semiconductorlayer 15 may be an n-type impurity-doped GaN layer.

A second semiconductor layer 19 is located on the first semiconductorlayer 15, and an active region 17 is interposed between the firstsemiconductor layer 15 and the second semiconductor layer 19. The activeregion 17 may have a multi-quantum well (MQW) structure in which one ormore well layers 17 b and one or more barrier layers 17 a arealternately stacked on top of each other. Here, the well layer 17 b hasa narrower band-gap energy than the barrier is layer 17 a. The welllayer 17 b and the barrier layer 17 a may be composed of anAlInGaN-based compound semiconductor. For example, the well layer 17 bmay be impurity-doped or non-doped InGaN, and the barrier layer 17 a maybe impurity-doped or non-doped InGaN or GaN. The second semiconductorlayer 19 may be p-type impurity-doped GaN.

A second electrode 21 may be formed on the second semiconductor layer19. The second electrode 21 may be a transparent electrode which allowslight to be transmitted therethrough. The transparent electrode 21 maybe composed of ITO. A bonding pad 23 may be formed on the transparentelectrode 21 for external connection. The bonding pad 23 may be composedof, but is not limited to, Cr/Au Ni/Au, or the like. Further, a firstelectrode 25 may be formed on an exposed region of the firstsemiconductor layer 15, which is formed by partially removing the secondsemiconductor layer 19 and the multi-quantum well structure 17. Thefirst electrode 25 may also be composed of, but is not limited to, Cr/AuNi/Au, or the like.

According to one embodiment of the present invention, the light emittingdevice includes at least one carrier trap portion 27 in at least onelayer within the multi-quantum well structure 17. The carrier trapportion 27 serves to trap carriers by taking the place of dislocationsin the multi-quantum well structure 17. For this purpose, the carriertrap portion 27 is configured to have a band-gap energy that graduallydecreases from a periphery of the carrier trap portion 27 to the centerthereof, as shown in FIGS. 3 and 4. FIG. 4 (a) is energy band diagramstaken along lines I(a)-I(b) and II(a)-II(b) of FIG. 3. The energydiagram taken along line II(a)-II(b) is substantially the same as thatof a general multi-quantum well structure consisting of a well layer anda barrier layer. However, in the energy diagram taken along lineI(a)-I(b), the band-gap energy gradually decreases towards the center ofthe carrier trap portion 27 in the well layer 17 b. Here, although thedecrease in the band-gap energy is shown as a straight line in FIG. 4(a), the is band-gap energy may decrease in a step shape as shown inFIG. 4 (b) or in a curved-line shape as shown in FIG. 4 (c) bycontrolling the temperature, pressure and flow rate of a source gas in achamber during growth of the well layer 17 b. For example, when thelayer including the carrier trap portion 27 contains indium, the carriertrap portion 27 may be configured such that the indium content of thecarrier trap portion 27 gradually increases from the periphery to thecenter thereof. A difference in indium content between the outermostregion and the center of the carrier trap portion 27 may be at least 2%or more.

Herein, the carrier trap portion 27 refers to a structure capable ofusing carriers which can be trapped and lost by the dislocations. Such astructure is not limited to a physical shape. In other words, accordingto embodiments of the invention, the carrier trap portion 27 may be aphysical shape or a quantum-mechanical energy state capable ofefficiently using the carriers which can be trapped and lost by thedislocations.

As described above, since the carrier trap portion 27 acts as theradiative center that traps carriers injected into the multi-quantumwell structure 17 to emit light, the carrier trap portion 27 accordingto the embodiment may be formed in the well layer 17 b within themulti-quantum well structure 17. For example, the carrier trap portion27 may be embedded in the well layer 17 b as shown in FIG. 2 or 3.However, it should be understood that the configuration of the carriertrap portion 27 in the well layer 17 b is not limited thereto.

Formation of the carrier trap portion 27 in the layer may be achieved bythree-dimensional growth. The three-dimensional growth refers to amethod of growing the layer in a three dimensional shape by controllingthe pressure, temperature and flow rate of a source gas within thechamber. For example, if the layer including the carrier trap portion 27is a well layer 17 b composed of InGaN, initial growth of InGaN iscarried out two-dimensionally until InGaN is grown to a predeterminedthickness or more, and is then changed to three-dimensional growth, bywhich the InGaN layer is further grown to a predetermined thickness ormore, so that the layer including the carrier trap portion according tothe embodiments of the invention can be formed by phase separationcharacteristics of indium. Further, when the indium content exceeds 5%and the growth temperature exceeds 600° C., indium is subjected to phaseseparation in the layer and exhibit an intensive tendency to form thecarrier trap portion 27 according to the embodiments of the invention.By this method, the carrier trap portion 27 can be formed simultaneouslywith the layer including the carrier trap portion while the layer isformed.

As shown in FIG. 6, however, the three-dimensional carrier trap portion27 induces compressive stress in the well layer, thereby causingdeterioration in crystal quality of the multi-quantum well structure 17.In this regard, a layer comprising aluminum (Al), for example, a layercomposed of an AlGaInN-based or AlGaInP-based compound semiconductor,may be formed as the barrier layer 17 a in the multi-quantum wellstructure to generate tensile strength capable of offsetting thecompressive stress, thereby solving the problem of deterioration incrystal quality caused by the compressive stress. According to oneembodiment, the Al content of the barrier layer 17 a is adjusted togenerate the same tensile strength as that of the compressive stressgenerated in the InGaN well layer 17 b.

Further, when the InGaN well layer 17 b is used along with a GaN barrierlayer 17 a, the lattice constant of the InGaN well layer 17 b becomessimilar to that of the GaN barrier layer 17 a due to the carrier trapportion 27, which is formed in the InGaN well layer 17 b to have agradually increasing indium content from the periphery to the center ofthe carrier trap portion 27, thereby solving the problem caused by thelattice difference between the InGaN well 17 b and the GaN barrier layer17 a.

As described above, the light emitting device according to theembodiment of the invention is configured to prevent a reduction ininternal quantum efficiency caused by the dislocations in themulti-quantum well structure 17. To this end, the carrier trap portions27 of this embodiment may be distributed at a higher density than thatof the dislocations in the layer including the carrier trap portion 27.

The carrier trap portion 27 may have a size of 1˜10 nm. Alternatively,the carrier trap portion 27 may have a size of 2˜5 nm.

When the multi-quantum well structure 17 contains indium, there can be aproblem of indium evaporation due to characteristics of indium. Toprevent this problem, the multi-quantum well structure 17 according toone embodiment of this invention may further include an indiumevaporation preventing layer (not shown) to prevent indium fromevaporating from the layer including the carrier trap portion 27.

According to another embodiment, the multi-quantum well structure 17 mayfurther include a carrier trap cluster that is formed by clustering atleast two carrier trap portions 27 described above. This configurationis clearly shown in a dotted part of FIG. 5 or in FIG. 6. The carriertrap cluster may be formed by adjusting the temperature, pressure, andflow rate of a source gas within a chamber during three-dimensionalgrowth of the layer including the carrier trap portions 27. When forminga well layer 17 b composed of, for example, InGaN, a pressure of 300torr or more in the chamber will lead to an intensive tendency tocluster the carrier trap portions 27 to form the carrier trap cluster.

The carrier trap cluster may be separated a distance of at least 20 nmor more from another adjacent carrier trap cluster. For example, thecarrier trap cluster may be separated a distance of at least 40˜120 nmfrom another adjacent carrier trap cluster.

Although some embodiments have been described with reference to theAlInGaN-based compound semiconductor in the above description, it willbe apparent to a person having ordinary knowledge in the art that thepresent invention can be applied to other compound semiconductors suchas AlInGaP-based compound semiconductors and the like.

As described above, according to embodiments of the present invention,the light emitting device includes a carrier trap portion 27 and/or acarrier trap cluster in a layer within a multi-quantum well structure 17to efficiently trap carriers, which can be trapped by dislocations inthe multi-quantum well structure 17, so that the carriers trapped by thecarrier trap portion 27 and/or the carrier trap cluster can be used forlight emission, thereby improving internal quantum efficiency of thelight emitting device.

Further, according to another embodiment of the present invention, themulti-quantum well structure 17 of the light emitting device isconfigured to generate tensile stress capable of offsetting compressivestress generated in the multi-quantum well structure 17, so that themulti-quantum well structure 17 has improved crystal quality.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed is to limit the claims to thespecific embodiments disclosed in the specification and the claims, butshould be construed to include all possible embodiments along with thefull scope of equivalents to which such claims are entitled.Accordingly, the claims are not limited by the disclosure.

1. A light emitting device, comprising: a substrate; a firstsemiconductor layer on the substrate; a second semiconductor layer onthe first semiconductor layer; and a multi-quantum well structurecomprising at least one well layer and at least one barrier layerbetween the first and second semiconductor layers, at least one layerwithin the multi-quantum well structure comprising at least one carriertrap portion formed therein, the at least one carrier trap portionhaving a band-gap energy decreasing from a periphery of the carrier trapportion to a center of the carrier trap portion.
 2. The light emittingdevice according to claim 1, wherein the carrier trap portion has aband-gap energy decreasing in a straight line shape from a periphery ofthe carrier trap portion to a center of the carrier trap portion.
 3. Thelight emitting device according to claim 1, wherein the carrier trapportion has a band-gap energy decreasing in a step shape from aperiphery of the carrier trap portion to a center of the carrier trapportion.
 4. The light emitting device according to claim 1, wherein thecarrier trap portion has a band-gap energy decreasing in a curved lineshape from a periphery of the carrier trap portion to a center of thecarrier trap portion.
 5. The light emitting device according to claim 1,wherein the carrier trap portion is formed in the well layer within themulti-quantum well structure.
 6. The light emitting device according toclaim 5, wherein the carrier trap portion is embedded in the well layer.7. The light emitting device according to claim 1, wherein the layercomprising the carrier trap portion comprises indium.
 8. The lightemitting device according to claim 7, wherein the layer comprising thecarrier trap portion comprises an AlInGaN-based compound semiconductoror an AlInGaP-based compound semiconductor.
 9. The light emitting deviceaccording to claim 7, further comprising: an indium evaporationpreventing layer in the multi-quantum well structure to preventevaporation of indium from the layer comprising the carrier trapportion.
 10. The light emitting device according to claim 7, wherein thecarrier trap portion comprises indium in an amount gradually increasingfrom the periphery of the carrier trap portion to the center thereof.11. The light emitting device according to claim 10, wherein the centerof the carrier trap portion comprises at least 2% or more indium than anoutermost region of the carrier trap portion.
 12. The light emittingdevice according to claim 7, wherein the layer comprising the carriertrap portion is the well layer and the barrier layer comprises aluminum.13. The light emitting device according to claim 12, wherein anAl-content of the barrier layer is adjusted to generate tensile stresscapable of offsetting compressive stress generated in the layercomprising the carrier trap portion.
 14. The light emitting deviceaccording to claim 12, wherein the barrier layer comprises anAlGaInN-based compound semiconductor or an AlGaInP-based compoundsemiconductor.
 15. The light emitting device according to claim 1,wherein the carrier trap portion is formed simultaneously with the layercomprising the carrier trap portion while the layer is grown.
 16. Thelight emitting device according to claim 1, wherein the carrier trapportions are distributed at a higher density than a dislocation densityof the layer comprising the carrier trap portion.
 17. The light emittingdevice according to claim 1, wherein the carrier trap portion has a sizeof 1˜10 nm.
 18. The light emitting device according to claim 17, whereinthe carrier trap portion has a size of 2˜5 nm.
 19. The light emittingdevice according to claim 1, further comprising: a carrier trap clusterformed by clustering at least two carrier trap portions.
 20. The lightemitting device according to claim 19, wherein the carrier trap clusteris separated a distance of at least 40˜120 nm from another adjacentcarrier trap cluster.